feat: add organization custom property resource

[oscarbc96]

Resolves #ISSUE_NUMBER

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Before the change?

• The github_organization_custom_property resource did not exist.
• There was no way to manage GitHub organization-level custom properties via Terraform.

After the change?

• Added a new github_organization_custom_property resource.
• Supports defining name, type, allowed values, default value, requirement status, and edit permissions.
• Includes full validation logic using CustomizeDiff to ensure consistent and correct configurations.
• Supports custom marshaling to allow default_value to be a string, list, or null depending on the property type.
• A full test suite was added to validate all logic paths and prevent invalid configurations.

Pull request checklist

• [x] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• [ ] Yes
• [x] No

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